1. Field of the Invention
The invention relates to the shaping of the gudgeon pin bores formed in gudgeon pin bosses of pistons for internal combustion engines or compressors.
When the piston is in use in an engine, the gudgeon pin bosses are subject to high stresses applied by the associated connecting rod through a cylindrical gudgeon pin which is fitted tightly in the bores. The gudgeon pin bosses hold the ends of the gudgeon pin while the connecting rod engages a central portion of the gudgeon pin. The maximum forces are generated during the firing stroke of the piston as a result of detonation of a charge in the associated piston. These forces urge the piston downwardly and this tends to bow or bend the ends of the gudgeon pin downwardly and force the centre of the gudgeon pin upwardly. This, in turn, causes the gudgeon pin and the gudgeon pin bosses to be forced into engagement with one another, particularly at the inner ends of the bores in those areas of the bore-defining walls of the gudgeon pin bosses which lie on the crown side of a plane which includes the common axis of the gudgeon pin bores and which is normal to the piston axis. The high stresses induced in these areas can lead to the appearance of cracks in the gudgeon pin bosses which start in these areas and rapidly spread. Such cracks cause the piston to fail and are therefore highly undesirable.
2. Review of the Prior Art
The existence of these high stresses and the appearance of cracks have been known for many years and there have been many previous measures directed to reducing their incidence. These measures are performed before the gudgeon pin is assembled onto the connecting rod with the gudgeon pin and comprise the formation of reliefs in those highly stresses areas where cracks are most likely to appear. The presence of a relief reduces the forces created between the gudgeon pin and the gudgeon pin bosses as the gudgeon pin bends or bows and so reduces the stresses and the incidence of cracks.
These reliefs have heretofore been formed by a machining process involving the cutting-away of the appropriate areas of the gudgeon pin bosses using, for example, a milling cutter. Another proposal has been to form the reliefs by roll-burnishing around the whole interior surface of the gudgeon pin bosses. Examples of these techniques are shown in U.S. Pat. Re. Nos. 26,764 and 4,189,932 and British Patent Specification Nos. 1,464,204 and 1,362,262.
These known techniques have the disadvantages that they require a separate machining step. In addition, the extent of the relief can only be guessed so that it is possible for the relief to be either excessive or insufficient, both of which are undesirable.